The present invention relates generally to a turbine flowmeter, and more particularly to a turbine flowmeter having a structure for developing viscous resistance and being capable of measuring not only low-viscous fluids but also high-viscous fluids having high viscosities, in particular, with excellent instrumental error characteristic.
A conventional turbine flowmeter is not accompanied by any problems in the case of measuring the flow quantity of a low-viscosity fluid such as water or gasoline but, in the case of measuring the flow quantity of a high-viscosity fluid such as heavy oil, is accompanied by difficulties such as large instrumental error and a narrow rangeability resulting therefrom, as will be described hereinafter.
In general, between the flow rate Q and the rotational angular velocity .omega. there is the following relationship, EQU (.omega./Q)=(tan .alpha./rA)-(Tf/r.sup.2 .rho.Q.sup.2)-(Tm/r.sup.2 .rho.Q.sup.2) (1)
where .alpha. is the angle of the turbine blades with respect to the pipe axis, r is the average radius of the blades, A is the area of the annulus defined by the inner and outer circumferences of the blades, Tf is the rotational resistance torque of the vane wheel or rotor caused by the fluid viscosity, Tm is the rotational resistance torque of the rotor caused by mechanical resistance, and .rho. is the fluid density.
Here, since rotational resistance torque Tm caused by mechanical resistance is negligibly small in comparison with the rotational resistance torque Tf caused by the fluid viscosity, the above described relationship may be considered to be as follows. EQU (.omega./Q)=(tan .alpha./rA)-(Tf/r.sup.2 .rho.Q.sup.2) (1a)
In the case where viscosity factor of the fluid is designated by .mu., the rotational resistance torque Tf is representend, depending on the flow conditions, as follows. ##EQU1## Accordingly, flow rate to rotational angular velocity ratio .omega./Q is represented as follows, ##EQU2## where K.sub.1, K.sub.2, and K.sub.3 are constants, respectively, and tan .alpha./rA is a specific value determined by design.
As will be observed from the Equations (5), (6), and (7), that .omega./Q is constant irrespective of the flow rate Q, in the case of turbulent flow, whereby no instrumental error occures. In the case of the laminar flow and transition flow, however, .omega./Q is a function of the flow rate Q, whereby the instrumental error is caused to vary depending on the flow rate. Accordingly, in the case of measuring a low-viscosity fluid by means of a known turbine flowmeter, there occurs little change in the instrumental error because the low-viscosity fluid assumes turbulence flow from a relatively low flow rate region in the flow rate measuring range. In contrast, in the case of measuring a high-viscosity fluid, there arises a difficulty in that the instrumental error changes greatly as the flow rate varies because the high-viscosity fluid flows in laminar flow state up to a relatively high flow rate region, and the turbulence flow region within the flow rate measuring range is thereby small.
Moreover, the flow velocity distribution is different in laminar flow, transition flow, and turbulence flow. Accordingly, the point of application of rotational torque acting on the blades changes according to the flowing state, which results in changes of the above mentioned average radius r, whereby the instrumental error is not constant. As a consequence, the known turbine flowmeter exhibits a large change in instrumental error in particular with respect to a high-viscosity fluid which flows in the state of laminar flow up to a relatively high flow rate region, whereby accurate flow rate measuring with wide range-ability cannot be carried out.
Furthermore, even in the case where the rotor is normally supported rotatably at opposite ends thereof in the known turbine flowmeter, the fluid impinging on the blades exerts a thrust on the blade wheel in the flow direction, and a part such as a bushing of the rotor makes contact with a bearing of the blade wheel and thereby imparts a complicated rotational resistance to the blade wheel thereby to cause the bearing to develop a large rotational resistance. As a result, the term of the mechanical resistance torque Tm in the Equation (1) is no longer negligible, which thereby generates a large measuring error in all flow rate regions. Another possible adverse result is uneven contact of the rotor shaft in a bearing, whereby the bearing wears rapidly until it becomes useless in a short time.